1. Field of the Invention
The present invention relates generally to the fields of cancer therapy and cancer prevention. More particularly, it concerns predicting taxane chemosensitivity in a cancer patient.
2. Description of the Related Art
One of the main problems associated with cancer chemotherapy is that individual patients with the same histology do not respond identically to a given agent or a given therapeutic protocol. The response range may vary in large proportions, even in chemosensitive tumors, such as breast cancer. A number of determinants of drug sensitivity are well known, such as drug dose, drug combinations and schedule of administration, patient age and status, tumor localization, etc., but the intrinsic sensitivity of a given tumor is a major factor which remains difficult to evaluate. Numerous mechanisms of resistance have been identified and some of them can be evaluated on tumor biopsies, but they do not solve the problem of tumor sensitivity to anticancer drugs. This has lead to a number of research to try to individualize drug treatment as a function of the sensitivity of tumor cells.
One approach to determine the sensitivity of anticancer agents on tumor cells obtained from patients involves an in vitro test, the Human Tumor Stem Cell Assay (HTSCA). This assay along with a variety of other assays developed, all involve four basic steps: 1) isolation of cells; 2) incubation of cells with drugs; 3) assessment of cell survival; and 4) interpretation of the result, which are all used to predict how effective the drug may be in a patient. However, these in vitro assays are often a poor predictor of chemosensitivity to anticancer agents in vivo. Currently, the standard protocol for chemosensitivity prediction testing is performed by an ex vivo test. In this protocol, surgically resected tissue is embedded in medium containing an anti-cancer drug, and incubated for one week. The size or viability of tissue is then measured, and the sensitivity determined. However, this method is time-consuming, completely manual and expensive ($600-1000 for one test). Thus far, chemosensitivity testing has not reached clinical use, and, presently, no successful prediction of sensitivity of a tumor to an anticancer drug in a given patient can be achieved routinely.
One group of anticancer agents for which the chemosensitivity needs to be determined involves the taxanes, such as paclitaxel. It has been indicated in the literature that paclitaxel resistance might be related to the spindle assembly checkpoint and Cdk1. When paclitaxel stabilizes microtubules and interferes with the dynamic changes that occur during formation of the mitotic spindle, the spindle assembly checkpoint is activated to make cells arrest at mitosis (Horwitz et al., 1982). The mitotic checkpoint/spindle assembly checkpoint, also known as the cell cycle, monitors accurate chromosomal segregation and plays a crucial role in maintaining genome homeostasis. This checkpoint monitors both the attachment of chromosomes to the mitotic spindle and the tension across the sister chromatid generated by microtubules to prevent premature chromosomal segregation.
The molecular components of the spindle assembly checkpoint were initially identified in Saccharomyces cerevisiae. Mammalian homologues of the checkpoint proteins include Mad1, Mad2, BubR1, Bub3, and Mps1 (Li and Benezra, 1996; Jin et al., 1998; Taylor et al., 1998; Chan et al., 1999). The checkpoint machinery is a protein complex composed of Mad1, Mad2, BubR1, Bub3 and cdc20, located at the kinetocore of the chromosome. The target of this checkpoint is the anaphase-promoting complex (APC) and its co-activator Cdc20. Mad2 and BubR1 are located downstream and appear to be the major proteins of this machinery, interacting with Cdc20 directly and inhibiting APC activity cooperatively (Fang et al., 1998; Sudakin et al., 2001; Tang et al., 2001; Fang, 2002).
In tumor cells, defects in the checkpoint are often observed, and these are believed to induce genome instability. Recently, Huang et al. (2000) suggested that MAD2 and CDK1 kinase are cooperatively involved in Paclitaxel-induced apoptosis. In other reports, activation of CDK1 was shown to be required for apoptosis induction through caspase-3 activation (Tan et al., 2002), and p21Waf1, a CDK1 inhibitor at M-phase, was demonstrated to play a key role in the inhibition of Paclitaxel-induced apoptosis (Fang et al., 2000).
Cdk1, combined with mitotic cyclins, is a universal master kinase required for regulation of mitosis (Nigg, 2001). Cdk1 activity is maximized in accordance with activation of the spindle assembly checkpoint. Previous reports using either a Cdk inhibitor or dominant-negative Cdk1 have shown that Cdk1 is critical for paclitaxel-induced cell death inhibitor or dominant-negative Cdk (Meikrantz and Schlegel, 1996; Shen et al., 1998); however, whether activation of Cdk1 is the cause or the consequence of activated checkpoint activation remains unclear. The relationship between the spindle assembly checkpoint and paclitaxel sensitivity therefore, remains unclear.
Thus, new and better approaches to the use anticancer agents such as taxanes in the treatment of cancer are needed. The effectiveness of these agents may be determined by assessing the chemosensitivity.